In magneto-optic recording, data is represented by a magnetized domain. As used herein, a magnetized "domain" is a stable magnetizable region having a preferential magnetized orientation. The preferentially magnetized domain is often referred to as a "bit." The preferential magnetized orientation is caused by an energy source, such as a laser, of sufficient intensity heating the medium above its Curie temperature while the medium is simultaneously biased by a magnetic field. The laser is used to heat the magnetic medium in the localized area. When the localized area exceeds the Curie temperature, the magnetization direction is set by the magnetic field.
When the laser beam is removed, the bit cools in the presence of the biasing magnetic field and has its magnetization switched to that direction. The momentary temperature rise in the bit reduces the bit's coercive force so that the magnetic biasing field forces the magnetization of the domain to conform to the direction of the biasing magnetic field as the bit cools below its Curie temperature.
To write again on the recording medium, conventional practice has been to erase what has been recorded. To accomplish the erasure, any given bit is exposed to a laser beam of sufficient intensity while also exposing that bit to a magnetic field in an opposite direction and permitting that bit to cool. This erasure step sometimes is referred to as an initialization step. The medium is then ready for writing on. Thus, the conventional write-over procedure requires two steps: a first erase or initialization step and a second recording or write-over step.
Various techniques have been proposed for making magneto-optic media directly overwritable. One is a hybrid optical/magnetic technique which relies on magnetic field modulation to write data on the medium. Another is a laser power modulation technique, in which the magnetic field is not varied, but instead, the laser power is modulated according to the input data stream between two different power levels. This latter technique has the advantage of being suitable for laser diode arrays, and therefore, in principle, it is capable of extremely high data transfer rates.
Exchange-coupled direct overwrite techniques make use of laser power modulation to determine the direction of magnetization of the written bit. In some prior art techniques, a large initialization magnet is required in addition to the smaller write magnet. In another technique, only the smaller write magnet is required. When the laser is pulsed to the low-write power level, a bit is formed in the storage layer. When the laser is pulsed to the high-write power level, the bit is formed in a lower bias layer and then copied into the storage layer as the film stack cools. Because the bit is formed initially in the bias layer during the high-write power operation, the magnetic properties of the bias layer must be carefully chosen to obtain well defined bit edges for copying.